1. Field of the Invention
The invention relates to an X-ray generator for powering an X-ray tube which comprises at least two electron sources for forming focal spots of different size at the same area of the anode.
2. Description of the Related Art
X-ray tubes capable of forming two (or more) focal spots at the same area of the anode are known (DE-OS 28 50 583). The known X-ray generators for powering such X-ray tubes enable formation of X-ray images with either a small or a large focal spot. X-ray images formed using the smaller focal spot have a better resolution, whereas X-ray images formed using the larger focal spot require shorter exposure times because the larger focal spot can be loaded more and hence generates more X-rays. Therefore, for an X-ray exposure the operator must each time decide which of the two focal spots is to be used.
In practice, however, some X-ray images require a resolution which is higher than can be achieved by means of the large focal spot and an exposure time which is shorther than can be achieved by means of the small focal spot, i.e. a higher power. Such images could be formed by means of an X-ray tube which comprises at least one further electron source whose focal spot dimensions are between those of the large and the small focal spot. However, such X-ray tubes are complex.
Furthermore, from U.S. Pat. No. 3,882,339 it is known to vary the size of the focal spot in a grid-controlled X-ray tube by varying of the voltage at the control grid. However, thus far such X-ray tubes are available for low power only.